This invention relates to a screening element for use in the construction of modular screening decks used for the screening of particulate materials.
It is known to manufacture a screening element, from a synthetic plastics material, with a plurality of screening apertures which are in communication with one another. This allows the apertures to expand and this, at least to some extent, prevents the blocking of the individual apertures by the material being screened and the blinding of the screening element.
The applicant is aware of a number of screening elements employing the general principles stated above. These known screening elements all have a plurality of rib formations which extend across the screening element. These rib formations are arranged side by side on the screening element to define the apertures between them. The apertures so formed are in a staggered configuration and each aperture is bordered by at least four sections, two each from two different rib formations. Each pair of sections is V-shaped and in combination forms a generally square aperture. In this manner the entire screening element is made up of apertures of the same square shape.
In order to screen the particulate materials effectively it may be beneficial to configure the rib formations in such a way that sections of the rib formations lie directly in the flow path of the particulate material. It is also important to control the rigidity and flexibility of the rib formations of the screening element.
The applicant is further aware that in certain screening elements that are manufactured from a synthetic plastics material the rib formations which define the apertures of the screening element are V-shaped in cross-section. In order to provide sufficient strength to the V-shaped elements substantial material is required.
It is also known, in order to provide strength to the plastics screen, to trap a metal frame within the plastics material. The metal frame is normally of mild steel and is formed from flat bar which is cut, bent and welded to form a frame of the required size and shape.
The manufacture of the metal frame is a laborious, labour intensive and multistep process. A large and robust frame adds significant weight to the screen.
During the manufacturing of the screen separate spacers are placed on the metal frame whereafter the plastics material is cast or injection moulded around the metal frame. In this manner the metal frame is trapped inside the plastics material.
As a result of the characteristics of the plastics material and the metal frame no direct bonding occurs between the plastics material and the metal frame. Heat from the plastics material, during the casting or injection moulding process, is transferred to the metal frame and if the size and width of the metal frame are not within specific parameters, warpage, bending and distortion of the metal frame occur as a result of the heat. In practice it has been found that a metal frame having a width of at least 8 mm and a height of 12 mm successfully withstands the heat from the plastics material. The minimum width of the metal frame results in broad sections of the screen having to be reserved for the metal frame which results in a loss of screening area.
The metal frame is further susceptible to oxidation in harsh environments.
During use of the screen, wear of the plastics material results in the eventual exposure of the metal frame. Once the metal frame is exposed the plastics material is easily dislodged from the metal frame which can result in the catastrophic failure of the screen.
During the recovery of a used screen it is also difficult and costly to recover the metal of the frame for re-use in subsequent metal frames.
It has further been found that when the screen is subjected to impact the metal frame often bends and distorts the screen. This could affect the productivity of the screen.
A further problem associated with all screens is the fact that if the screen is dislodged from a fixed structure supporting it the failure of the screen often goes unnoticed which results in defective screening taking place. If the dislodged screen is transported away from the screening area substantial production time could be wasted in the recovery of the screen and the re-screening of the affected material.